In general, a fuel cell comprises a cell on which an air electrode layer, an electrolyte layer and a fuel electrode layer are laminated, a fuel-supplying part for supplying a fuel as a reducing agent to the fuel electrode layer and an air-supplying part for supplying air as an oxidizing agent to the air electrode layer, and it is an electric cell in which electrochemical reaction takes place between a fuel and oxygen in the air in the cell to bring out an electric power to the outside. Fuel cells of various types are developed.
In recent years, because of a rise in an interest to environmental problems and energy saving, it is studied to use a fuel cell as a clean energy source for various applications. In particular, attentions have been paid to fuel cells which can generate electric power by only supplying directly a liquid fuel comprising methanol and water (refer to, for example, patent documents 1 to 3).
Among them, liquid fuel type fuel cells making use of capillary force for supplying a liquid fuel are known (refer to, for example, patent documents 4 and 5).
Liquid fuel type fuel cells described in the above respective patent documents, so-called passive type fuel cells assume a system in which a fuel is supplied to a power generation cell by virtue of capillary force and in which a pump for supplying a fuel, valves and the like are not required to be provided, so that it is a fuel supplying system which enables to expect an electric generator to be reduced in a size.
In the development of a direct methanol fuel cell as an electric source for small-sized portable appliances, there has been involved a problem of “methanol-crossover” in which methanol is transmitted through an electrolyte membrane as it is unreacted to bring about a loss of a fuel and reduction in electrode performances. As a result, a concentration of the fuel used is reduced to 1 to 3 M (mol/liter), and energy density of the fuel cell system is reduced as well to a large extent as compared with the theoretical value (4800 Wh/liter).
Countermeasures on systems such as development of an electrolyte having a low methanol-crossover and supply of high concentration methanol to an electrode after it is diluted with water to control a concentration have so far been employed as a measure for solving the above problem.
However, it is the existing situation in the former case that in the development of the electrolyte having a low methanol-crossover, a practical membrane making use of high concentration methanol of 10 M or more has not yet been found. On the other hand, in the latter case where a fuel is prepared by diluting high concentration methanol with water, the system is complicated since a methanol concentration sensor, a diluting device, an alcohol tank provided in combination with a water tank and the like are required, and involved therein is the problem that the system capacity grows larger in proportion thereto to reduce energy density in the whole equipment.
On the other hand, the present inventors proposed a fuel cell in which a porous carbon plate is used for a support, a power collector, a liquid supply and gas discharge medium, wherein capillary force of the porous carbon plate is used (refer to, for example, patent document 6).
The fuel cell of the above structure has characteristics and performances which have not so far been observed, but local unevenness is produced in discharge resistance when the produced carbon dioxide passes through the porous body from the electrode surface in power generation. Further, there is a part in which a methanol fuel in the form of a liquid is brought into contact with the electrode membrane, so that methanol-crossover can not be reduced to a large extent in a certain case when using a liquid fuel such as methanol having a high concentration exceeding 10 mol/L and the like, resulting in small output. Therefore, it involves some problem that large output can not be obtained.    Patent document 1: Japanese Patent Application Laid-Open No. 258760/1993 (claims, examples and others)    Patent document 2: Japanese Patent Application Laid-Open No. 307970/1993 (claims, examples and others)    Patent document 3: Japanese Patent Application Laid-Open No. 313047/2001 (claims, examples and others)    Patent document 4: Japanese Patent Application Laid-Open No. 188008/1994 (claims, examples and others)    Patent document 5: Japanese Patent Application Laid-Open No. 93551/2001 (claims, examples and others)    Patent document 6: Japanese Patent Application Laid-Open No. 251715/2005 (claims, examples and others)